Many types of electric motors, such as dynamo-electric machines and stepper motors, utilize laminated metal parts. These parts, such as rotors, stators, and cores, comprise a number of thin metal laminae. The laminae are stamped from a thin sheet of metal and stacked in alignment to form the desired part.
In stamping out the laminae for rotors and stators, it is important that corners and edges of the teeth be sharp, clearly-formed, and of consistent dimension. If the corners of each tooth are not clearly defined, or the outlines of the teeth are inconsistent from lamina to lamina as many laminae are stamped out, the laminae will not stack properly and the motor will be noisy and inefficient. Therefore, it is important that the teeth in each lamina be stamped within very fine tolerances and have sharp, clearly-defined corners and edges.
It has long been a problem in the art to stamp out teeth of small size and accurate shape without damaging the part, for instance by bending the teeth in the course of stamping.
In the case of, for example, the stepper motor stator shown in FIG. 1, there are two parameters which must be carefully controlled in order to produce a satisfactory lamina. The first is the inner diameter formed by the outermost edges of the inward-facing teeth 18. Because a rotating part will be moving within the space defined by the inner diameter, it is important that the inner diameter be correct and uniform from lamina to lamina. The other important parameter is the shape of each of the teeth 18. If the teeth do not form sharp, consistent shapes when the laminae are stacked, the motor will be noisy and inefficient.
In the stamping art, there are generally two standard practices to overcome the problems of stamping out small teeth without damaging the metal. One of these practices is to stamp the teeth in one die station and the inner diameter in another die station within the same die. In effect, the second die clips each tooth down to the appropriate inner diameter. This procedure has an inherent shortcoming, in that use of more than one die station requires particularly careful alignment and coordination of the cut of each die. Sometimes, in order to stamp small teeth without bending them, half the teeth are stamped in one station, and the other half are stamped in the following station, followed by the cutting of the inside diameter. Such a procedure not only increases expense, but introduces another possible source of inaccuracy.
Another type of manufacturing process commonly used is to stamp the edge of the lamina and all the teeth at the same time, using a spline punch and die ring. This method avoids the problems of aligning two die stations but has its own drawback: it is impossible to create a sharp corner on a tooth tip since, as a practical matter, a perfectly sharp corner on an inside corner of a spline cannot be formed. Best efforts have achieved a .003 inch radius but this still varies from between 0.005 to 0.007 inch on the finished part due to wear and material break when sheared.
Various methods for cutting or stamping teeth in a lamina under the exacting conditions required for electric motors are described in other patents. Gerstle U.S. Pat. No. 3,834,013 shows a sequence of steps for cutting out a lamina for a stator. First a circular opening that will correspond to the inner diameter of the stator is cut. Then, a plurality of openings are cut around the rim of the circular opening. These openings will become the interstices between the teeth of the stator. Finally, openings are cut connecting each of the smaller openings to the central circular opening, so that the interstices are fully defined. According to the specification of the Gerstle patent, at column 4, lines 15-17, the tips of the teeth must be trimmed in a shaving operation to conform to the intended inner diameter of the stator.
Mitsui U.S. Pat. No. 4,280,275 (of which U.S. Pat. No. 4,438,558 is a continuation) shows another sequence for cutting teeth in a stator lamina. First, a plurality of openings forming a circular array, representing the interstices of the teeth, are cut in a thin metal sheet. Then, a central section representing the inner diameter is blanked out.
In the methods of both of these patents, teeth with well-defined edges are produced, but because the interstices and the inner diameter are produced in separate steps, and the large inner portion defining the inner diameter is removed from the metal after the interstices are cut, the stamping force to remove the inner portion places stress on the teeth, causing them to bend or be otherwise damaged. Consequently, there is a limit on how small the teeth of the stator can be made. If the individual teeth are too small, they may well be damaged by the stamping force in blanking out the central portion, or in cutting the slots to the central portion.